The proposed research has three objectives: First, study the effect of genes at the B blood group-histocompatibility locus and the L blood group locus on the regression of Rous sarcoma virus induced tumors in backcross progeny produced from matings of F1's of inbred lines to the pure parental lines. Second, investigate whether or not B locus genes in our stocks influence the mixed leukocyte reaction. Third, continue studies with acridine orange and immunofluorescent stained lymphocytes to identify the red fluorescing material, and investigate serum proteins and membrane proteins of lymphocytes with emphasis on B locus antigens.